Compression ignition engines, such as diesel engines, produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). As emissions standards for permissible emission of pollutants from compression ignition engines, particularly vehicular engines, become progressively tightened, there is a need to provide improved catalysts and exhaust systems that are able to meet these standards and which are cost-effective.
Oxidation catalysts comprising platinum group metals have been used to treat carbon monoxide (CO) and hydrocarbons (HCs), including the volatile organic fraction (VOF) of particulate matter (PM), in exhaust emissions produced by compression ignition engines. Such catalysts treat carbon monoxide (CO) by oxidising it to carbon dioxide (CO2), and treat hydrocarbons (HCs) by oxidising them to water (H2O) and carbon dioxide (CO2). Some platinum group metals, particularly when supported on a refractory oxide, can also promote the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2).
It has been found that platinum (Pt) and palladium (Pd) are each able to oxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine. Palladium is generally cheaper than platinum, but is less active toward CO and HCs (e.g. Pd has a higher light-off temperature for CO and HCs than Pt). Palladium is also more susceptible to poisoning by sulfur in fuel compared to platinum. However, platinum-based oxidation catalysts have been found to generate nitrous oxide (N2O) by reduction of NOx (Catalysis Today 26 (1995) 185-206).
Current legislation for regulating engine emissions does not limit nitrous oxide (N2O) because it is regulated separately as a greenhouse gas (GHG). Nevertheless, it is desirable for emissions to contain minimal nitrous oxide (N2O). The US Environmental Protection Agency has stated that the impact of 1 pound of nitrous oxide (N2O) in warming the atmosphere is over 300 times that of 1 pound of carbon dioxide (CO2). Nitrous oxide (N2O) is also an ozone-depleting substance (ODS). It has been estimated that nitrous oxide (N2O) molecules stay in the atmosphere for about 120 years before being removed or destroyed.
Typically, an exhaust gas of a compression ignition engine will be treated using an exhaust system where an oxidation catalyst has been combined with at least one other emissions control device. In general, the emissions control device will not remove any nitrous oxide (N2O) generated by the oxidation catalyst, even when the emissions control device receives treated exhaust gas from an outlet of the oxidation catalyst (i.e. the emissions control device is downstream of the oxidation catalyst).